1. Field of the Invention
The present application relates to a pressure-sensitive adhesive composition.
2. Discussion of Related Art
A liquid crystal display device (LCD) is a device that displays an image through liquid crystal present between two transparent substrates.
When a voltage is applied to the LCD through electrodes connected to the liquid crystal, a molecular arrangement mode of the crystal display is changed, and thus the transmissivity of light is varied, thereby displaying an image.
Basically, the LCD includes two transparent substrates, a liquid crystal panel including liquid crystal between the substrates, and a polarizing plate attached to the transparent substrate of the liquid crystal panel.
The polarizing plate includes an iodine-based compound or a dichroic polarizing material arranged in a certain direction, and has a multilayer structure which includes a protective film to protect a polarizing element formed on one or both surfaces thereof. Also, the polarizing plate may further include a retardation film having a unidirectional molecular arrangement, or a wide viewing-angle compensation film such as a liquid crystal-type film. Respective films constituting such a multilayered polarizing plate are made of materials having different molecular structures and compositions, and thus show different physical properties. As a result of difference in contraction or expansion behavior of materials having a unidirectional molecular arrangement, dimensional stability is insufficient, especially under high-temperature or high-temperature/humidity conditions. Therefore, when the polarizing plate is fixed by means of a pressure-sensitive adhesive, stresses are concentrated on a TAC layer due to the contraction or expansion under high-temperature or high-temperature/humidity conditions, which leads to problems such as birefringence and light leakage.
A representative method used to solve the above problems includes a method of designing a pressure-sensitive adhesive to have a stress relaxation property so that the pressure-sensitive adhesive can be easily crept and deformed with respect to external stress. For example, Korean Patent Publication No. 1998-79266 and Japanese Patent Publication Nos. 2002-47468 and 2003-49141 disclose a method of mixing a high molecular weight polymer that has a functional group which can react with a cross-linking agent, with a low molecular weight polymer that has a small amount of a cross-linkable functional group or no cross-linkable functional group.
However, the pressure-sensitive adhesive compositions disclosed in the patent documents have problems in that creeping or squeezing of a pressure-sensitive adhesive may be caused due to poor cuttability in manufacture of a polarizing plate, which leads to a decrease in yield.
Another method used to prevent light leakage includes a method of designing a very hard pressure-sensitive adhesive. Such a pressure-sensitive adhesive may serve to minimize stress by suppressing the contraction and expansion of a polarizing plate under high temperature or high-temperature/humidity conditions and concentrate the stress on the outermost layer of the polarizing plate, which allows the attainment of superior optical properties.
In order to realize a hard pressure-sensitive adhesive, the modulus of elasticity of the pressure-sensitive adhesive needs to be increased. However, as the modulus of elasticity of the pressure-sensitive adhesive increases, the durability may be degraded due to an accompanying decrease in strength of the pressure-sensitive adhesive. Also, the use of a common single cross-linking structure makes it difficult to achieve a modulus of elasticity at which light leakage is suppressed and durability is sufficient. Accordingly, a method of forming a separate cross-linking structure by adding a photoinitiator and a multifunctional acrylate to a pressure-sensitive adhesive composition used to form a conventional single cross-linking structure is also known in the art.